In order to produce MRT images having an optimum signal-to-noise ratio (SNR), reception antennae may be placed in close proximity to an object to be measured (e.g., a patient or subject). Such reception antennae are known as local coils. The local coils may be in the form of reception coils but may also be transmission coils.
A local coil is a physical unit that may be used by an MRT user. The physical unit may have a plurality of logical subunits (e.g., coil elements) that the user may individually select or deselect for a measurement. A coil element in turn may combine a plurality of reception antennae (e.g., local coil antennae) that produce the individual MRT signals that are ultimately used in the MRT imaging as single and independent signals. By way of example, a local coil may have three individually addressable coil elements. Each of the three individually addressable coil elements may have six individually addressable reception antennae. Thus, in this example, a local coil may have 18 single reception antennae.
A multiplicity of reception antennae may provide faster MRT measurements through parallel imaging methods. A higher antenna density of reception antennae (e.g., the number of reception antennae of the local coil) corresponds to higher image quality. However, the multifold reception antennae produce many independent signals that may be transmitted via numerous cables and digitized by many analog-to-digital (AD) converters.
A switching matrix may be used between the reception antennae and the AD converters to make at least one selection of signals, such that the number of AD converters is limited to an amount n (e.g., the maximum number of reception antennae that may be used at one time). The amount n is limited by the magnitude of the homogeneity volume of the MRT. In practice, not all of the coil elements may simultaneously come to be in the homogeneity volume of the MRT.
If, for reasons of cost, a system having fewer than n receivers (e.g., A/D converters plus infrastructure) of the image-processing unit is used, an unaltered set of local coils may not allow the entire homogeneity volume of the MRT to be used for imaging. There are an insufficient amount of receivers of the image-processing unit available for image processing. Conventional local coils are not compatible with a small number of receivers of an image-processing unit for image processing. As a result, the signals from the reception antennae may be customized in a scaling unit prior to transfer to the image-processing unit. In addition, a plurality of local coils may be used in an MRT. The plurality of local coils may all be connected to the scaling unit.
DE 10313004 B3 describes a combination network that allows a combination of single signals from a multiplicity of reception antennae. The combination network is provided with a multiplicity of inputs and an identical number of outputs. All the inputs are connected to all the outputs within the combination network. Within the combination network, the single signals from the multiplicity of reception antennae may be weighted and/or phase shifted.
The data signals from the local coils are redistributed by the combination network. The original data signals from the antennae are applied to the input, and the signals that are referred to as “modes” are applied to the output. With an appropriate design of the combination network, the signal at output 1 may provide a sum of all the antennae (e.g., “CP mode”) and at least geometrically cover the reception range of these antennae (e.g., as a single larger virtual antenna). The additional signals from the higher modes may be mixed from the original signals, such that the additional signals add new information content to the respective previous modes. Thus, combinability of the local coil is achieved. The use of a single reception circuit allows signals from the entire geometric range to be used, and the use of further reception channels allows the use of further modes, thereby improving image quality and facilitating parallel imaging methods, for example.
The combination network described in DE 10313004 B3 is complex and expensive to design, manufacture, maintain and repair. In addition, individual tuning may be used during production.
Although the modes hold the same information content as the original signals when all of the modes are used and the combination network is correctly implemented, the signal characteristic is substantially altered. Moreover, the use of the higher modes in unaltered algorithms for MRT image calculation may be problematic.